Since I can't switch sims bt 900 and 4S, is there a way to get email notifications on the 900? Reason I ask is we have a 7-mo old, and today she needed something and my wife had trouble getting a hold of me.
Here is what I know about the sim. When I got my Lumia 800 I had to have a micro sim for it. I got one and used a sleeve adapter with the micro sim with my Focus S. I swapped that sim between the L800 and the Focus S a lot no issues. When I got the Lumia 900 the guy in the store told me that the sum I have was the same as the one that came in the box. I am now using that sim in my 900 and everything works fine I have not swapped the sim to any other phone sense putting it in my L900. I think it will work fine if you swap it but I haven't tried it.
You must be busy with your 7-month-old because your posts are unclear.
If you can move your SIM, use an old-fashioned method: have your wife call you.
Subject to the usual (there-are-no-free-markets) locking limits, your SIM is portable and allows you to receive calls (only) on whichever phone it is installed; data service is separate and tied to your/a phone, and atandt has to switch/set that to one phone or another, with each switch.